Process for intensifying silver photographic images



Patented Dec. 8, 1953 PROCESS FOR INTENSIFYING SILVER PHOTOGRAPHIC IMAGES Jacob Quentin Umberger, Holrndel, N. J assignor to E. I. du Pont de N emours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application July 17, 1952, Serial No. 299,497

8 Claims. 1

This invention relates to p o p ye particularly it relates to a process for intensifying weak silver photographic images. Still more particularly it relates to a process for intensifying silver photographicimages by toning the silver image with a plurality of polyvalent metal salt solutions.

In processes of color photography, for example, it is desirable to increase the optical density of silver images for various purposes and particularly for printing purposes. Thus, it is often desirable to obtain stable images which have densities 8 to 10 times the density of the original silver image. Uranium toned images have been proposed for this purpose, but uranium salts are not available in large quantities for this use. Iron toned images show a large degree of intensification but are low in optical density to the blue light normally used for printing.

An object of this invention is to provide a process for providing toned silver images. Another object is to provide such a process that will produce stabletoned silver images of high density. Yet another object is to provide such a process which utilizes available chemical raw materials.

A further object is to provide such a process which can be carried out in a continuou manner without careful observation and involved technical procedures. Still further objects will be apparent from the following description of the invention.

It has been found that controllable increases, up to ten-fold, and even higher in density or contrast of photographic silver images can be obtained by treating such an image in a Waterpermeable colloid layer of an exposed and developed photographic element, e. g., a film or paper, with an aqueous solution containing ferricyanide ions and one or more cations taken from the group consisting .of lead, zinc, silver, mercury, copper, nickel, cobalt and cadmium followed by treatmentwith a second aqueous solution containing one or more inorganic polyvalent metal cations that form relatively water-insoluble ferrocyanide compounds and one or more anions taken from the group consisting of blue-light-absorbing complex oxide ions of hexavalent chromium.

The lead, zinc, silver, mercury, copper, nickel, cobalt or cadmium ions in the first bath are cations which form relatively insoluble ferrocyanide and chromate compounds. Best results are obtainable with lead ions. Hence, it is preferred to use lead ions alone or in preponderating amounts when zinc or otherions are present.

When silver images are'toned by the procedure just described, the resulting toned images markedly are increased in optical density to blue-light and, depending on the particular cations in the second solution, to all visible wave lengths.

. In the preferred aspect of the inventionthe silver image in the exposed and developed photographic element is first converted into a lead ferrocyanide image by treating the silver image with a lead ferricyanide bleach bath, for instance, an aqueous solution containing a water-soluble,

inorganic lead salt, an alkali metal'ferricyanide and an alkali metal bromide. The photographic element is then washed with water and the white image of Pb2Fe(CN) e is converted through a metathetical reaction into a mixed image com posed of yellow lead chromate and blue ferric-,

is not intended to be limited bythe following examples.

Example I A photographic film consisting of a colorless cellulose triacetate film base having a thickness of approximately 0.0055 inch provided With a thin gelatin sublayer was coated with a gelatino silver bromoiodide emulsion which contained about 96.0 mols of AgBr per 3.4 mole of AgI to produce a coating Weight of approximately 9 mg. silver] halide and 67 mg. of gelatin per square decimeter.

The resulting photographic film was exposed in an intensity-scale sensitometer (seeC. E. K. Mees The Theory of the Photographic Process,

- MacMillan 1948, pp. 607-623) utilizing a simulated daylight source and developed for five minutes inthe following developer at 68 F.:

Water cc 975.0 Mono-methyl-p-aminophenol,sulfuric acid salt gm 0.8 sodium sulfite, anhydrous gm 90.0 Hydroquinone gm 1.0 Borax gm 3.0 Potassium bromide gm 0.1

After fixing, washing and drying, the silver image wasjobserved to beef very' low optical density. On measurement by an electronic 3 densitometer it was found that the maximum diffuse density was 0.16. Then this silver image was bleached for five minutes at 68 F. in a solution of the composition:

Water ml 700.0 Acetic acid, glacial ml 30.0 Lead nitrate ems 16.0 Potassium ferricyanide, dissolved in 100 cc. I

H2O gms 8.0 Potassium bromide, dissolved in 100 cc,

H2O gms. 2.9

Water to make 1.0 liter.

The resulting bleached film was washed for five minutes with agitation in water at 58 'F., and then treated for ten minutes at 68 F. in a solution of the following composition:

Water ml 700.0 Ferric nitrate 9H2O gms 40.0 P tassium. d c r at -sm. Potassium ferricyanide gms, 7.5

Watertomake 1.0 liter.

The resulting. film was. washed for minutes and dried and it was found upon measurement that the diffuse optical density to blue light (400- 490), millimicrons transmitted bya filter was increased from 0.16to. 2.0. The density of the intensified. image was. equivalent to that of a normal, negative, film having a Weston exposure speed, of approximately 40, and. the gamma was approximately 1.1 toblue. light. To green and red, light the gammas were approximately 0.8 and 1.1, respectively.

A control film made from the same emulsion but, having a coating weight of. 85 mg. silver halide perv square. decimeter produced a silver image characteristic. of: light sensitive film having Weston speed 30. andgamma 0.6 when developed forfive minutesin the above developer and fixed, washed and dried.

Er m al acetate having. a medium viscosity mixed with 4; mg of 99% hydrolyzed. polyvinyl acetateof medium. viscosity per. square decimeter. Thestripping layer. was coated with agreen-sensitive gelatino silver. bromoiodide photosensitive emul sion to produce acoating weight;of approximately 70mg. silver, halide and 70mg. gelatin per square decimeter. A .water -sensitive stripping layer having a coating weight of 21 mg. per square decimeter and composed of mediumviscosity 88% hydrolyzedpolyvinyl acetate was coated onto the latter green-sensitive gelatin silver,bromoi odide layer. The upper stripping layer was .coatedwith a blue-sensitive gelatino silver bromoiodide emul sion of the type and having a,.coating weightas set forth in Example I. A suitable dye for the green-blind emulsionv is 1,1'-diethyl-2,4-carbo cyanine iodide and for. the green-sensitive one is 1,1f -diethyl-2,2'-cyanine iodide. 1

The film element just described was exposed to a multicolor scene and after wetting it with water at a temperature of F. the outer blue and green record emulsion layers were successively transferred to separate blank films consisting of a cellulose acetate film base having a layer of gelatinat pI-I'a at coating weight of 70 mg. per square decimeter. After drying the separate films bearing the red and green records they were developed to gamma 0.75 in a develcper solution having the composition given in Example I, fixed, washed and dried. The remaining blue record film (bearing the low coating weight emulsion) was developed to completion in a developer of the same composition given in Example I, fixed, washed and dried. The weak silver image of the blue record emulsion was bleached to completion at 68 F. in a solutionof the following composition:

Water ml 700.0 Acetic acid, glacial ml 30.0 Lead nitrate gms 16.0; Potassium ferricyanide, dissolved in cc.

H2O gms 2.9

Water to make 1 .0 liter.

washed: for a periodof fiyeminutes in, cold water; with-agitation and then tonedto gamma approxi mately 0.8 to green light in a solution of the following composition;

Water ml 700;0 Ferric nitrate 91120 gms 40.0 Potassium dichromate gms 3.8- Potassium ferricyanideflu gms 7.5

Water to make 1.0 liter.

The three separate film elements containing the color separation records were then separately printed into the. appropriate layers of a multilayer color print film of the type descrilz ed in the Journalof the Society of Motion Picture and Television; Engineers, November 1950, pp.

455-476. This exposed multilayer film was thenprocessed to a color reproduction as describedin idi malg.

The presence of ferricyanideions inthe second ortoning solution is not essential but does pro duce a higher degree of intensification. The mechanism of this efiect isnotknown with certainty. A magenta or green light-absorbing. dye orpigment e. g. Anthraquinone Rubine R cone, (Colour- Index 1091), Pontacyl Carmine 63 Extra (Colour Index,, ,57) and Pontacyl Ru,-

bine. Extra, conc. (Colour IndexTl78) could be placed in, the final .rinse to at least partially neutralize the green color-of themixed image of. lead chromate andferric.ferrocyanide. The dyesjust mentioned are. described: in the 19511Technical; Manual and Yearbook; of the American Association of; Textile Chemists, and Colorists, vol. 27, Howes Publishing Co., Inc., New York, N Y.

Exa-mpZeJIL The processset forth in Example I" was re..

peatedexceptthat the film had the silver halide coating weight increased to 17 'mg, per square decimeter, This filmhad a maximum diifuse optical density of 0.32 when developed. to asilver image. When intensified by the procedure of Example I but replacing the ironsaltin the toner by 20 gms. uranyl nitrate per liter, the maximum,

diffuse density was increased to 213 to .blue light.

In the place of the lead nitrate in the fore- "going first or bleach bath'there may be substithe first bath of the foregoing examples there may be substituted various other acids in equivalent amount. Suitable acids which can be substituted or admixed with the acetic acid are formic acid, hydrochloric acid, citric acid, nitric .acid, glycollic acid and lactic acid. The amount of acid can be varied over fairly wide limits depending on the desired degree of acidity and/or sequestration. Buffering salts such as alkali acetates, formates, lactates, citrates, etc. can be added to the solution for pH regulation in the preferred range of l to 6.

Likewise, in place of the potassium ferricyanide used in the foregoing first bath, or in addition thereto, one may substitute other equivalent ferricyanides including ammonium ferricyanide, so- 'dium ferricyanide, and mixed ferricyanides of such cations. The ferricyanide compounds can be used in amounts varying from 0.5 to 25 grams per liter of solution.

All or part of the potassium bromide can be omitted or substituted by other agents which form insoluble silver salts, such as sodium and ammonium chlorides and bromides can be used in amounts of 1 to 25. grams per liter of solution.

Suitable zinc and cadmium salts which can be substituted in equivalent amount for all or part of the lead salts include zinc and cadmium acetates, zinc and cadmium sulfates, zinc and cadmium nitrates, and zinc and cadmium chlorides. The corresponding soluble salts of silver, mercury, copper, nickel, and cobalt can also be used in equivalent amounts.

With regard to the second toning bath various other water-soluble chromate or dichromate salts can 'be substituted for all or part of the potassium dichromate. Among such other salts are: sodium and ammonium chromates and dichromates. The proportion of the chromate or dichromate salts may vary from 0.5 to 25 grams per liter.

In place of the ferric salt or uranyl salt used in the. second bath one may substitute other polyvalent salts containing in solution the following cations that produce insoluble ferrocyanide compounds: VO++, Cu++, Ag+, Co++, Mn++, I-Ig++, Ni++. These insoluble ferrocyanide compounds are usually colored but need not necessarily be colored. For example, a colorless ferrocyanide compound co-precipitated with lead chromate can add to the printing density of the latter by making the path of blue light through the film more devious. Furthermore, the presence of a white pigment in the film can lessen grain patterns by its diffusing and scattering action on light.

The amount of the intensification of the image can be controlled by varying the amount of time the film element is in the first and second baths. The first bath essentially is a bleach bath and the second a toning bath. The time of treatment in these baths may vary from 1 to 20 minutes. Ilhe temperatures may vary in the baths but, in general, should be maintained from 50 to 80 F. For'constantum'form results, it is best to bleach to completion in the first bath and then tone the bleached image in the second bath. The bath 'of Example I above is a fast-acting toner. A.

practical slow-acting toner solution is as follows.

Water 'c 700 Acetic acid, glacial cc 3 FeCl3.6HzO gms a K2CrO4, dissolved in cc. water gm Water to make 1.0 liter.

As evident from this solution and Example I, the

molar concentration of ferric salt may vary from- In some films the gelatin coating weight is insuificient to prevent loss of lead chromate and Prussian blue by leaching from the film during the intensification procedure. In this event it is necessary to omit the potassium bromide from the bleach bathand/or reduce the amount of potassium ferricyanide.

- The high degree of intensification attainable through employment of this invention makes practicable the use of 'bipacks and tripacks hitherto handicapped by (l) halation from the front film support which could in the past 'be eliminated only by antihalation tints which reduce emulsion speed, (2) low resolution in rear films due to poor contact between front and rear films during exposure, and (3) low photographic speed; For certain applications of photography it is sometimes desirable to place one film in front of another without appreciably decreasing the intensity and resolution of the light image recording in the rear filml For example, bipack photograhy has been used in color photography and also can be used in methods for forming silhouette masks for use in combining background and foreground scenes of different origin.

If a film as low in silver halide coating weight as that of Example I, or even lower, is used as afront fihn of an extended bipack film (such as the stripping rear film disclosed in Example I of Umberger U. S. application Ser. No. 274,287, filed February 29, 1952) there is little loss in sharpness and speed in the rear film. For color work, one can obtain seperation negatives of surprisingly good quality by using atripack composed of a very low coating weight blue record film in frontof a bipack of green and red record films held in emulsion-to-emulsion contact. The

base thickness interposed between the blue record emulsion and the contact point of the green and red record emulsions would normally seriously affect the image quality of the latter records. With approximately 5 mg. of silver halide per square decimeter in the front blue record film, however, adequate image quality is obtained in the green and red records. Following processing, the front film is intensified as described above and in Example I.

Another use of the procedure of this invention is the intensification of radiographs. 1 The invention can be used for the intensification of images in various types of photographic films including the negative extended bipack films of Jennings U. s. Patent 2,462,503, .the stripping films of Marasco U. S. Patent 2,182,814, Rackett U. S. Patent 2,415,442, and Schule U. S. Patent 2,448,552, and the front film of bipacks exposed in beam-splitting cameras such as described on p. 753 in Neblette Photography Principles and Practice, 4th ed., Van Nostrand (1942). The films of these references contain various natural and synthetic water-permeable colloids e. g. gelatin as binding agents in the image bearing layers.

zaeeama The stripping and transfer *operations-toan "the ca'rrie'd out'von' vanyof the conventional: apparatus used for registration and image transfer, e...g., .the apparatus described in .the following U. S. ;,.patents: Wyckofi et al. 303,836 ant1'1.,303837, zBall .1 ,66 1,157, Thornton 13301291, "Whitney 1,707,699, Comstock 1,707,710, R'ackett 22711572, Rackett 2,369,176 and 2,4I;442,"Capsta1r'2,411060 -and"Conklin 2,578, 5I4. V

"The -var-ious advantages of this invention reside in the fact that controllable increases, up "to ten-Told, inthe' density or contrast are-ob- *ta'ined. Another advantage is that readily avail- "able materials are used. further advantage i's "that intensified images which are stable and "ofgood color can be obtain'ed.

Another advantage of the invention" is :that it becomes possible to obtain normal speed and gamma (about 028) from 'silver hali'd'e coating weightsas-lowas 5 to mgrpersquare decime- 'ter. With-coating-weig-ht as low'as-thi's': in the blue record-of 'color camera negative films, very high' 'speed and resolution result in the underlying g-reen and red record emulsion layers.

"Grain size is increase'd by the intensification 'processo'f-this invention but probably the grain -i's di ffused during printing. Thus, when printing the toned negative with blue light, the-lead *chromate grains-absorb the light. "The Prussian 'Blue -g-rains -are essentially colorless to blue light *and' scatter 'or diffuse the -grainy pattern produeed by' the' lead chromate particles.

many' widelydifferent-embodiments of this invention can be made Withoutdeparting from "the-spirit and-scope thereof;- it is to be understood that the invention is not 'to' be limited except as defined by the claims.

"-What is claimed is:

1. A process for intensifying 'a silver photographic image Whichcomprises treating a photo- 'graph'ic "element bearing a silver image in a -waterpermeable "layer thereof, in an aqueous =solution containing ferri'cyanide ions in an amountcorrespondingto 05 to 25.0 grams per liter-ofa' soluble fer'rlcyanide compound and at least one-*cation which forms relatively insoluble ferrocyanid'e and chromate compounds in an" amount equivalent to 1.0 to --25.-0--grams' per liter oflead =nitrateand treating the resulting relativelyinsoluble ierrocyanide salt image in an' aqueo'uss'olution containing at least one polyvalent metal-cation which formsa relatively water-insoluble ferrocyan-ide compound in an amount equ-ivalent to? 0.01 to 0.1- molar concentration offerric salt and at least one blue-lightabsorbing complex oxide anion 0f hexavalent chromium in 'an'amount-equivalent to 0.5 to 25.0 grams per liter-of sodium chromate.

2. A process for intensifying-silver v(photographicimages which comprises treating a photographic element'bearing a silver image. in 'a '-water-=permeable layer thereo't'in' an. aqueous solution '-containing ferricyanide .;ions in an amount -corresponding to 0.5 toj.0. grams per liter? of a" soluble. ferricyanide compound. and at least one'ca-tion takenifromithe-group consisting of lead, zinc, silver, mercury, copper, nickel cobalt, and cadmium ions luntil. said: silver image is. converted to .a. metal ferrocyanideimage of .the latter metals, said cation being .presentin an .amount equivalent to- 1.0 to 25.0 grams per liter of lead .nitrate, andtreatingthe. resulting elementiin an aqueous solution containing .at "least one pblyvalent metal. cation which forms a :relatively water-insoluble ferrocyanijde nom- :poun,d =in;an;amount equivalent to 10.0.1 tOfOel molar concentration :of :ferric salt and at least one blue-lighteabsorbmg "complex oxide anion of hexavalent chromium .in an-amountequi-va- :lent to v0.5 :to -25;0 grams per liter of {sodium chromate.

3.A process for intensifying -silver photo- :gr-aphic 'images whichrcomprises treating -:-a, photographic (element g-bearing a silver ,image in 78- waterepermeable layer thereof, -in an :aqueous solution ;containi-ng 'ferricyanide ions in an amount corresponding to 0.5 to 25.0 grams ;-per liter ;of a -soluble;--ferri.@.yanide compound and lead cations inan amount equivalent to to 25.0 grams pergliter -of; lea-d nitrate, until the silver image is converted :to a leadjferrocyanide image,

.and treating the: resulting elementin :an aqueous 5SOIUtiOHIJCQHtQiI'li-TIQI atleast one poly-valent metal cation which forms a relatively .Waterinsoluble ferrocya-nide compound vin an amount equivalentto 0:01 to 0'.1-molar concentration of ferric salt a-nd "at least one blue-light-absorbing -,com -plex-'oxide--.anion :of :hexavalent chromium, in an amount gequivalentetot05 to 2530 "grams per liter of :sodium; chroma-te.

4. A process for intensifying silver photographic images which comprises treating :aepho- 'togra-phic .=elem'ent bearing-:a silver image in a water-permeable@layer' thereoiiin an aqueous solution containing terricyan'ide :ions in :an amount corresponding 130.1035 25:0. grams-- per liter, of a soluble ierricya-ni'de compoundand lead cations. in ail amount .--"equivalent to l;0 to 125.0 --g:rams-;per:liter -of lead --nitrate, until the silver image .is convertedaiio a: lead ferrocyani'de-image, and treating the; resulting elementin anaqueens-solutioncontaininga soluble ferric saltwhich .forms :a relatively water-insoluble ierr'ocyan'ide nompound: in an; amount :equivalent to .0;01 ;,.to 0.1 molar concentrationeof 'ferric salt and an :alkali--m'eta-l ;-dichromate an amount of 10.5 to .251) grainsperzliter.

:5. A" rprocesszias 286i) forth' in -;claim =4 rwherein .the:= 1ead-:sa1t1 is-.-lead nitrate and the "ferric salt is ferric nitrate.

-6. -A wp-i'ocess for intensifying silver photographic images: which comprises: treating. a =pho- .tograph-ic :e1ement bearing .a silver image :in a wa-ter-pe,rmeable vlayer thereof, in an aqueous solution containing pferricyanide ions in an .amount;- corresponding 12005 to 25.0 grams per liter-of asoluble -ferricyanid'ecompoun'd an'dlead cations .in-anaamoun't equivalent to 1.0 to 25.0 -grams; per-.l-iter; of lead nitrate'until the silver image .is -.converted to a le ldaferrocyanide. image, and treatinglthe resulting element in. an aqueous --solution containinga soluble ferric salt which .forms. a relatively. water-insoluble ierrocyanide compound in an amount equivalent .to 0.01 to -0.1 molar concentration of ferric salt andanalkali -metal chromatelin an amountof 0.5 to 25.0 grams 'perliter.

7. A process for intensifying silver photographic images which comprises treating the'phoitographicfilm' element bearing a silver image in awater-permeable colloid layer thereof, in an aqueous-solution containing acetic acid, lead injtrate-in-.an amount-of 1.0.to'25.0 grams-per liter potassium iferri'cyanide in an amount of 0.5 to 25.0 grams per ili'ter andzpotassium bromide in an amount of 11.0 to"25:.0 grams. per. liter until the silver.,image is-converted 'to.-a lead .ferrode mas e and r atinegthe. resultin film .elementvin an, aqueous.- solution containing-ferric nitrate in 0.01'to 0.1 molar concentration of ferric salt, potassium dichromate in an amount of 0.5 to 25.0 grams per liter and potassium ferricyanide in an amount of 0.5 to 25.0 grams per liter. i

8. A process for intensifying silver photographic images which comprises treating the photographic film element bearing a silver image in a water-permeable colloid layer of a said phographic element, in an aqueous solution containing acetic acid, lead nitrate in an amount of 1.0 to 25.0 grams per liter, potassium ferricyanide in an amount of 0.5 to 25.0 grams per liter and potassium bromide in an amount of 1.0 to 25.0 grams per liter until the silver image is converted substantially completely to a lead ferrocyanide image, and treating the resulting film element in an aqueous solution containing ferric nitrate in an amount of 0.01 to 0.1 molar concentration of ferric salt, potassium chromate 20 10 in an amount of 0.5 to 25.0 grams per liter and potassium ferricyanide in an amount of 0.5 to 25.0 grams per liter.

JACOB QUENTIN UMBERGER.

, OTHER REFERENCES Clerc: Photography, Theory & Practice, Pitman, N. Y., 2nd ed. 1937, pp. 383-386 (paragraphs 595, 596, 600) 

1. A PROCESS FOR INTENSIFYING A SILVER PHOTOGRAPHIC IMAGE WHICH COMPRISES TREATING A PHOTOGRAPHIC ELEMENT BEARING A SILVER IMAGE IN A WATER-PERMEABLE LAYER THEREOF, IN A AQUEOUS SOLUTION CONTAINING FERRICYANIDE IONS IN AN AMOUNT CORRESPONDING TO 0.5 TO 25.0 GRAMS PER LITER OF A SOLUBLE FERRICYANIDE COMPOUND AND AT LEAST ONE CATION WHICH FORMS RELATIVELY INSOLUBLE FERROCYANIDE AND CHROMATE COMPOUNDS IN AN AMOUNT EQUIVALENT TO 1.0 TO 25.0 GRAMS PER LITER OF LEAD NITRATE AND TREATING THE RESULTING RELATIVELY INSOLUBLE FERROCYANIDE SALT IMAGE IN AN AQUEOUS SOLUTION CONTAINING AT LEAST ONE POLYVALENT METAL CATION WHICH FORMS A RELATIVELY WATER-INSOLUBLE FERROCYANIDE COMPOUND IN AN AMOUNT EQUIVALENT TO 0.01 TO 0.1 MOLAR CONCENTRATION OF FERRIC SALT AND AT LEAST ONE BLUE-LIGHTABSORBING COMPLEX OXIDE ANION OF HEXAVALENT CHROMIUM IN AN AMOUNT EQUIVALENT TO 0.5 TO 25.0 GRAMS PER LITER OF SODIUM CHROMATE. 